Some stock market participants may be jittery about geopolitical turmoil and the possibility of a Federal Reserve rate hike sooner than expected.

But Jeremy Siegel, a finance professor at University of Pennsylvania, is sticking to his bullish guns. Since January he has forecast that the Dow Jones Industrial Average will hit 18,000 by year-end, and now he says it may even be 19,000.

The market now trades at 16.5 times 2014 earnings, Siegel said. A move to 18,000 would imply a price-earnings ratio of 17.5, "a very reasonable multiple given the interest rates," he said.

As for potential pitfalls for stocks, "we live in a world of uncertainty, and bull markets climb the wall of worry," Siegel said. "When we see nothing in the future that can worry us at all, I'll get worried, and I'll probably tell people to sell stocks."